The present invention relates to a method of making a chain, in particular a round steel chain of tempered steel.
Chains are assembled of single links and used as pulling elements, driving elements or conveying elements. In general, link chains which have single links that interlock for spatial movement, are distinguished from articulated chains which have individual links that are interconnected by bolts for rotation movement in a plane. Link chains are made of drawn wires or rolled round steels on an industrial scale. Therefore, they are referred to as round steel chains and used for lifting and moving loads as well as for securing a shipment and also for transporting bulk material. In view of their spatial mobility, they are especially suited for use in underground coal mining.
The demands on used materials are essentially characterized by the interaction of high strength (hardness) and minimum values for the notch impact energy to thereby ensure a high wear-resistance and resistance to fracture when exposed to operational stress.
Further requirements are dictated by the applied manufacturing process. This involves the use of steels that are suitable for electric resistance welding processes, in particular for flash-butt welding. This requirement necessarily results in a limitation of the carbon equivalent.
The currently governing steel standard for welded round steel chains is reflected in DIN 17115 from the year 1987. This standard relates to special steels with specified contents of manganese, chromium, nickel or molybdenum as well as limited values of phosphor and sulfur. A typical member of this material group from which currently high-quality chains are usually made involves a steel of grade 23MnNiMoCr54. The narrow analytic limitations as well as the regulations with respect to mechanical-technological values should ensure a high uniformity of the finished chains. Modified materials may be used for particular purposes. Examples of additional alloy elements include vanadium, tungsten or titanium.
After hardening, the steels are normally annealed at a temperature range of about 500xc2x0 C. resulting in tensile strengths of up to 1,250 MPa in conjunction with notch impact energy values of at least 60 J. After annealing, the chain is calibrated in a stretching operation to thereby realize the demanded chain geometry and to improve the precision fit, in particular with respect to the interaction of the chain with drive wheels. The enhancement of the chain through calibration is, however, accompanied with a work-hardening and a drop in the notch impact energy capacity. The work-hardening is considerably more pronounced in chains of higher strength than in standard chains. A problem during operation arises, when the drop in notch impact energy is about 10 J to 15 J in chains of higher strength and the hereby associated susceptibility to brittle fracture.
From the standpoint of the user, a further increase of the chain strength is demanded while still preventing a decrease in resistance to brittle fracture. A chain of steel having notch impact energy values between 35 J and 40 J after annealing would be unsuitable in practical operation, especially for the use in coal mining
It would therefore be desirable and advantageous to provide an improved method of making a chain to obviate prior art shortcomings and to produce a chain with a tensile strength of above 1,550 MPa and a notch impact energy of at least 55 J.
According to one aspect of the present invention, a method of making a chain, in particular a round steel chain of tempered steel, includes the steps of combining chain links to produce a chain strand, subjecting the chain strand to a heat treatment, calibrating the chain strand, and subjecting the chain strand to a thermal aftertreatment at a temperature between 190xc2x0 C. and 250xc2x0 C.
According to another feature of the present invention, the thermal aftertreatment may be implemented at a temperature between 210xc2x0 C. and 240xc2x0 C. e.g. at about 230xc2x0 C.
The present invention resolves prior art problems by providing a thermal aftertreatment after calibration. Through the provision of the thermal aftertreatment, stress resulting from the final calibration process is relieved. While the tensile strength drops as a consequence of the thermal aftertreatment as does also the yield point, the notch impact energy increases, however, to values above 55 J. The tensile strength still remains above 1,550 MPa despite the drop as a result of the thermal pretreatment. The toughness of the chain is derived from a residue in austenite in the microstructure of the tempered steel and is responsible for the increase in notch impact energy during the thermal aftertreatment. The invention thus departs from the current teaching to avoid austenite in steel because of its highly adverse affect in traditional special steels to decrease strength.
According to another feature of the present invention, the initial heat treatment is implemented by the steps of normalizing, hardening and annealing. Annealing is implemented at a low temperature level under 200xc2x0 C., e.g. 190xc2x0 C. The tempered steel used has a microstructure with a stable residual austenite content of between 3% and 10% after hardening and annealing. After the final calibration process for realizing the chain link geometry and thus the precision fit, the chain strand is subjected to the thermal aftertreatment at a temperature between 190xc2x0 C. and 250xc2x0 C.
Normalizing may be carried out at a temperature between 900xc2x0 C. and 1,100xc2x0 C., followed by cooling with air. Thereafter, the chain strand is hardened at a temperature between 900xc2x0 C., and 1,000xc2x0 C. followed by quenching with water. Annealing is realized at a temperature between 180xc2x0 C. and 200xc2x0 C. at a retention time of typically four hours and by cooling with air.
According to another feature of the present invention, the chain strand may be tempered immediately after the calibrating step (main calibration), and recalibrated after the tempering step and before the thermal aftertreatment step. The main calibration is thus carried out prior to tempering. Tempering is followed by a recalibration to provide the final chain geometry. A distortion that may be caused during the hardening operation is thus eliminated. Thereafter, the thermal aftertreatment is carried out.
The thermal aftertreatment not only results in an increase in notch impact energy but also in a drop of the modulus of elasticity of the chain. As a result, the chain exhibits a softer spring characteristic. This has practical advantages, in particular in underground coal mining, because an increase in face length is accompanied by an ever increase in the length of the chains which are accordingly sensitive to vibrations. The vibrations can be compensated by the soft spring characteristics of a chain made by the method according to the invention.
According to another feature of the present invention, the tempered steel may be based on a steel alloy containing, by weight percent, between 0.20% and 0.27% of carbon (C), between 1.90% and 2.00% of chromium (Cr), between 1.30% and 1.70% of manganese (Mn), with copper fractions (Cu) between 0.20% to 0.50%, nickel fractions (Ni) between 0.15% and 0.40%, aluminum fractions (Al) of up to 0.03%, molybdenum fractions (Mo) of up to 0.05%, titanium fractions (Ti) of up to 0.04%, niobium fractions (Nb) of up to 0.04%, and silicon fractions (Si) of up to 0.05%, with the phosphor fraction below 0.01% and the nitrogen fraction (N) below 0.02%. Preferably, the copper fraction (Cu) may be between 0.45% and 0.50%. Also the nickel fraction may preferably be at an upper limit of the analytic range, i.e. for example at 0.35% to 0.40%.
According to another feature of the present invention, the tempered steel may have a chemical composition as mass parts in percent (%) as follows: 0.23 C, 0.05 Si, 1.50 Mn, xe2x89xa60.01 P, xe2x89xa60.004 S, 1.95 Cr, 0.37 Ni, 0.02 Mo, 0.028 Al, 0.5 Cu, 0.02 Ti, 0.024 Nb, 0.0105 N.
A chain made from such tempered steel is annealed during the heat treatment preferably at a temperature of about 190xc2x0 C., and during the thermal aftertreatment at a temperature of about 210xc2x0 C. Practical tests have shown that such a chain reliably attains a tensile strength of 1,600 MPa and more, with notch impact energy values between 55 J and 65 J. Load changes of more than 1,000,000 can be realized in fatigue tests.
According to another feature of the present invention, a tempered steel may be based on a steel alloy having the following composition, by weight percent, to produce a chain strand with very good mechanical properties: 0.18% to 0.24% of carbon (C), 1.60% to 1.80% of chromium (Cr), 0.75% to 1.00% of manganese (Mn), 0.50% to 0.85% of nickel (Ni), 0.20% to 0.30% of molybdenum (Mo), 0.01% to 0.10% of titanium (Ti), 0.20% to 0.40% of silicon (Si), 0.015% to 0.03% of aluminum (Al), and 0.001% to 0.0035% of boron (B). The phosphor fraction (P) is below 0.025%, and the nitrogen fraction (N) does not exceed 0.010%, in particular is smaller than 0.003%. The balance is formed by iron (Fe), including melting-based impurities.